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With the onset of the coronavirus and the international 'lock downs' I have started a daily blog from my solitary confinement here in the wilds of Normandy. 

 

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The roof article has languished for a long while now - but things are moving.  I HAVE managed to finish a slideshow about the caravan repair - under Renovations and DIY, which is also availabl on YouTube.  The roof renewal is due a similar treatment.

 

The 'Recipe' section has grown - and is growing.  Much tastiness to be found there!

 

Finished frontThis is the front of the house after the roof on this side had been finished.  It doesn't yet have the ridge tiles fitted, as they go on after the other side has been done.

As this took up so much of our time last year, I thought I might just run through some of the why's and wherefore's.  The trouble with doing these one off jobs (and no - I do not intend to repeat the project), is that you amass a great deal of knowledge and experience - and never use it again!  So maybe this might help somebody, somewhere, somewhen (as they used to say in Ringwood).

There are many different types of roof structure but I thought I might run through a few as the differences are considerable.  It will also help understanding the photos that we took, which follow.  Our house was built sometime in the 17th century and in those days they used what was to hand - granite for the walls and oak for the timbers as they grow everywhere here.

roof trusses

Modern french roof construction - for all those lovely magnolia 'pavillons' that you see everywhere - is just like that used in the UK - pre-stressed trusses, assembled off-site and erected very quickly.

They have some disadvantages - loft access can be tricky for storage etc - and loft conversion even more so, as all the timbers play a significant, structural part to the whole and altering them is a job for the experts.

rafters

Historically simple UK roofs were constructed as shown here, so that the common rafters carried a substantial load and were therefore quite substantial timbers.  They took the weight of the slates or tiles, supported by purlins, if necessary.

Some houses are still built this way today but the contruction method is considerably more expensive - both in terms of material and the skilled trades needed to build them.

One big advantage is the ease of conversion - there are no obstructions in the roof space and few structural timbers need cutting (apart from window installation for example).

truss

Our roof follows the style of most of the old houses round here - with massive oak king post trusses.

The roof and attic are a straightforward 'toblerone' shape - a gable at each end with 3 trusses at equidistant intervals between them.  No valleys, hips or dormer windows.

The trusses carry 2 purlins (les pannes) each side and the rafters (les chevrons) are fixed to the ridge, purlins and wall plate.  The chevrons are much lighter than UK rafters, as the trusses and purlins carry much more of the weight.

The chevrons tend not to run from wall plate to ridge beam, but are split into three parts. 

1 - wall plate to first purlin;

2 - first purlin to second purlin; and

3 - second purlin to ridge beam.

Finding a 5 metre straight run of light timber would have been nigh on impossible back in the day (and prohibitively expensive).  What original oak chevrons that remain (and there are many) whilst flat, are far from straight, following some interesting s shaped curves!  This can make nailing on the roofing battens later a little interesting.

There is good standing headroom right through the attic.  On the outside the roof measures just over 5 metres from ridge to gutter and 16 metres in length.  Altogether there were 162 m² (81m² per side) of roof surface to renew.

Roof structure

This photo taken in the attic (with half the slates off) gives a clear picture.  Luckily the weather was kind to us - and we picked our days carefully.

If you look at the top part of the roof (where there are still battens and slates), you'll see they didn't use roofing felt when the roof was last reslated.

Roofing or sarking felt is a fairly modern invention.  You'll find explanations as to how it prevents moisture ingress should rain or snow be driven under the slates and there is some truth in this.  In reality it 'dries in' a very large area of roof, very quickly.  Far more quickly than the nailing of battens and the laying of tiles would allow.

Modern materials allow for far more sophisticated felt than the tar impregnated stuff I remember from years ago.  Attics used to very very draughty places - deliberately.  This allows for the ready dispersal of moisture.  Eaves were not enclosed to encourage a good through flow of air.  These days houses are far more air and water tight.  That in turn has led to problems with damp and mould.

Which meant far more attention is now given to ventilation and moisture extraction.  Breathable membranes have now replaced the tar paper of old.

The oak king post truss can be seen clearly.  These are original i.e dating from the 17th century.  Some of the purlins have been replaced (the two nearest the camera for example) as have many of the original chevrons.  If you are looking at roof interiors, you'll often see worm holes in these old timbers.  Chemical treatments were unheard of back then, so the way our ancestors ovecame the problem was to build in oversized timber.  As oak ages, it dries and hardens.  It hardens to a point where you simply cannot drive a nail into it - you have to drill a pilot hole.  This also forms a problem for the wood boring insects.  The wood dries to a point where they simply cannot thew though it.  (Another reason to keep damp at bay).

I've offered people my penknife when they have expressed concern about woodworm holes in old attics.  Never once has the blade penetrated more than 10mm if that.  If you have ever cut one of these beams in half, you'll know how hard they become.

A frame

One of the areas where I have seen problems created, is where DIYers, with seemingly no appreciation of structural engineering, do an attic conversion in a roof with A frame construction.

Here the roof triangle is not braced at floor level, but halfway up like the letter 'A'.  The problem arises because the cross beam is now at waist height and not at floor level.  

What's the DIY solution if you want to do an attic conversion?  Just cut it out! 

What happens then?  As there is nothing bracing the legs of the triangle against the considerable weight of the roof they spread, often pushing out the tops of the walls.  I've pulled up at houses, looked up at the eaves and seen bowing and cracking.  I knew exactly what I would find on the inside.

(A brief aside.  Many years ago I was helping out a friend who was building an extension to a thatched house in Dorset.  The thatcher was a Lancastrian - if memory serves an ex-miner - who had moved south and changed occupation for a better life.  He was gently rustling away above our heads as we were inside looking at the pre-stressed trusses used for the new roof.

The site carpenter was with us, a highly skilled man but of an age where he wasn't too impressed with these flimsy looking new fangled contraptions.  "I'm telling you," he said "put any weight on those legs and they'll open up!"

Down through the thatch, in broadest Lancastrian floated the following;  "Eeh - I knew a lass like that once!"  Not much work got done for the next 15 minutes!)